<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button id="btn">发送ajax请求</button>

    <script>
        btn.onclick = function () {
            var xhr = new XMLHttpRequest;

            xhr.open('get', 'http://localhost:3000/error');

            // 发送请求
            xhr.send();
            xhr.onload = function () {
                console.log(xhr.responseText);

                if (xhr.status == 400) {
                    // 具体的出错信息，根据项目来定
                    alert('请求出错');
                }
            }

            // 当网络中断时，会触发onerror事件；
            xhr.onerror = function () {
                let a = '网络中断，无法发生ajax请求';
                alert(a);
                console.log(a);
            }
        }
    </script>
</body>

</html>